FIG. 9 is a schematic representation of prior art peristaltic pump 10. Pump includes drip chamber 12 connected to source of fluid 14, tubing 16 connected to the drip chamber, upstream valve 18 to block or allow fluid flow from the drip chamber to the tubing, and a plurality of fingers 20 to create a moving zone of occlusion along the tubing and to push the fluid downstream past downstream valve 22. The downstream valve is used to block or enable fluid output, for example, blocking fluid output when the upstream valve is opened. The upstream valve, fingers and downstream valves are typically engaged with cam lobes 24 on cam shaft 26 and rotated by motor and associated gears 28. Different shapes for the cam lobes determine the timing of opening and closing of the upstream and downstream valves and peristaltic function of the fingers. In general a pumping cycle for the pump is as follows: a first cam lobe operates the upstream valve to fully open the upstream valve to admit a volume of fluid while a second cam lobe operates the outlet valve to close the downstream valve; the first cam lobe operates to close the inlet valve and the second cam lobe operates to fully open the downstream valve; a set of cam lobes operates on the fingers such that the fingers expel fluid past the downstream valve; the second cam lobe operates on the downstream valve to close the downstream valve; and, the preceding sequence is repeated.
A typical maximum volume for the tubing between the upstream and downstream valves is 0.160 milliliter. Often the maximum volume is reduced due to the fingers partially compressing the tubing, for example, the maximum volume is reduced to 0.080 milliliter. Since the upstream and downstream valves and fingers are all mounted on a single cam shaft, it is not possible to control the amount of fluid entering the tubing independently from the movements of the fingers. The amount of fluid entering the tubing from the drip chamber is controlled by the opening and closing of the upstream valve, thus is not possible to control the amount of fluid entering the chamber, independently from the movements of the fingers. That is, for each pump cycle, the upstream valve is fully opened and an amount of fluid equal to the maximum volume flows to the tubing from the drip chamber. Therefore, the minimum amount of fluid entering the pumping chamber at each pumping cycle is 0.080 milliliter in this case.
Certain infusion regimens require very low flow rates, for example, 0.1 microliter/hour. Pump 10 has difficulty in maintaining flow continuity at such low flow rates.
Cam shaft 26 is supported proximate each end by respective bearings 30. The bearings hold the shaft in a position that is fixed except for rotation of the shaft. The fixed position is such that cam lobes 24 are able to operate fingers 14 and to open and close the upstream and downstream valves. In general, the cam lobes are positioned such that one of the upstream or downstream valves is closed at all times. One possible mode of failure for pump 10 is the failure of some or all of bearings 30. For example, the bearings can fail such that the shaft is no longer held in the fixed position noted above and one or both of the ends of the shaft are further from body. In this case, the cam lobes may be far enough from the fingers and/or the upstream or downstream valves such that the cam lobes are no longer able to close the upstream and/or downstream valves. Thus, for failure of some or all of the bearings, pump 10 is unable to control flow from the drip chamber. For example, in the sequence noted above, when the upstream valve is opened it is presumed that the downstream valve is closed. However, if the bearing failure results in the cam shaft being unable to close the downstream valve, an uncontrolled flow from the drip chamber results when the upstream valve is opened. An uncontrolled flow condition can be extremely hazardous to a patient receiving an infusion via pump 10, for example, resulting in a dangerously high dosage of a drug being infused with pump 10.
FIG. 10 is a schematic representation of a prior art gravity-feed infusion arrangement. In some clinical applications, fluid delivery by gravity, as shown in FIG. 10, is acceptable. For delivery by gravity, gravity force is strong enough to cause fluid to flow from container 32 hung on pole 34 through tubing 36 to the patient. However, the flow rate from container 32 cannot be automatically controlled and it is difficult to accurately control the flow rate. For example, roller clamp 38 is used to manually control the flow. The clamp is equipped with roller 40 that may be rolled by hand to contract tubing 14 to compress the tubing to control the flow through the tubing from container 32. Such manual control is not accurate and is very susceptible to human error.